fixes error messaging - physical resource id is always required

[vmadura]

Fixes #107

Tested using same steps in the reported issue and confirmed that correct error message is being reported:

[codecov-io]

Codecov Report

Merging #108 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #108   +/-   ##
=======================================
  Coverage   71.34%   71.34%           
=======================================
  Files          17       17           
  Lines         698      698           
=======================================
  Hits          498      498           
  Misses        156      156           
  Partials       44       44

Impacted Files	Coverage Δ
cfn/wrap.go	90% <100%> (ø)	⬆️

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[bmoffatt]

Thanks for the fix!